The present invention relates to rechargeable battery cells of high electrochemical charge capacity and, particularly, to such battery cells which function by reversible intercalation/deintercalation or other interchange of multivalent yttrium ions between cell electrodes.
Commercially successful rechargeable ion intercalation batteries have been based in great measure upon lithium wherein the negative cell electrode provides a source of lithium ions and comprises lithium metal, a lithium alloy, or, when the cell is in the charged state, a lithium-intercalated material, such as carbon. A combination of such lithium ion source electrodes with positive cell electrode materials capable of intercalating lithium ions, and suitable electrolytes and ion-transmissive, electron-insulative inter-electrode separator materials comprises a stable electrochemical cell which may be continually recycled between charged and discharged states to store and yield electrical energy for myriad electrical devices.
As simply viewed, the energy storage capability, or electrolytic capacity, of such cells is primarily dependent upon the numbers of ions, and thus the numbers of electron transfers, which can be derived from the available amount of active electrode materials for rechargeable cycling between cell electrodes. While a cell""s capacity may, generally, be increased simply by increasing the amount of active electrode material, practical cell weight limitations determine the feasibility of such an approach. Thus, the extensive use of lithium in rechargeable-battery cells reflects the low molecular weight of that material which enables the incorporation of considerable amounts of active ion source while maintaining the advantageously low weight of a resulting battery. A detraction from the desirability of lithium remains, however, in that the specific capacity, or the amount of electrical energy which may be stored and recovered from a given weight of active electrode material, is limited by the singular valence of lithium which yields only a single electron/ion transfer per unit weight of lithium.
Attempts have been made to increase the specific capacity of rechargeable battery cells by incorporating multivalent electrode ion source materials capable of yielding multiple electron/ion transfers per unit weight during electrolytic cycling of a cell. Investigations into the use of such materials are described in U.S. Pat. Nos. 5,601,949 and 5,670,275 which suggest the possible utility of alkaline earth and lanthanide metal compounds as intercalatable ion sources. Unfortunately, however, such materials of reasonable unit weight are limited to those of bivalent alkaline earth ions, such as calcium, magnesium, and strontium. The mentioned exotic trivalent lanthanide compound sources, such as lanthanum, europium, and samarium, not only lack practical availability, but they exhibit such high molecular weights that any specific capacity advantage anticipated due to trivalent ions is all but lost as the result of the increase in cell weight due to the heavier incorporated active electrode materials.
As a result of the present invention, on the other hand, the discovery of the utility of yttrium compounds as suitable active electrode materials for rechargeable battery cells provides such materials in a practical weight range which yield trivalent ions and extraordinarily increase the specific capacity of resulting battery cells.
In the investigations underlying the present invention, it has been found that, contrary to the expectation of a limited ability to diffuse and intercalate, alloy, or otherwise physically site into known receptor electrode materials due to the high charge density and normally strong intrastructural bonding of the relatively small trivalent yttrium ion, such ions are indeed capable of reversible insertion or intercalation with a number of metal oxide and sulfide receptor electrodes to a degree sufficient to support a practical, high capacity rechargeable electrolytic battery cell.
When combined in the usual manner with an ion receptor counter-electrode material and an electrolyte, e.g., a solution of a dissociable yttrium compound in a non-aqueous solvent, an yttrium-ion source electrode material comprises an electrolytic battery cell with a specific capacity in the range of up to about 250 to 350 mAh/g which may be repeatedly cycled between charged and discharged states to yield stable electrical current over a range of about 2 V. The structure of such battery cells may follow the mechanical style wherein layers of an ion source electrode material, such as yttrium metal, and receptor electrode material, such as a vanadium oxide, are compressively assembled with an interposed separator layer of an electroninsulating, ion-transmissive material, such as a borosilicate glass paper or a porous polyolefin membrane, typically saturated with a fluid electrolyte, such as a 1 M solution of yttrium perchlorate in an equipart mixture of ethylene carbonate and dimethyl carbonate. A commercially available Swagelok test cell is typical of such a mechanical structure and provides a simple apparatus for investigating the efficacy of proposed battery cell components and compositions.
A more commercially adapted and preferred cell fabrication comprises electrode and separator layers of active electrode compounds and electrolyte dispersed in polymeric matrix compositions which enable the lamination of those layers into a unitary, flexible electrolytic battery cell sheet, such as described in U.S. Pat. No. 5,460,904.
Ion receptor positive electrode materials which may be effectively employed in the present invention to achieve the noted improvements over prior electrolytic cells include, preferably in nano-material form, i.e., in a particle size less than about 500 nm, transition metal oxides and sulfides such as vanadates, e.g., V2O5, V6O13, or VO2, chromates, e.g., Cr3O8, molybdates, e.g., MoO3, tungstates, e.g., WO3, titanium disulfide , and yttrium silicide, which latter material may be utilized, as well, in negative ion-source electrodes with an appropriate positive electrode comprising, for example, V2O5. Alternative negative electrode materials include yttrium metal and alloys, such as yttrium aluminide, and yttrium carbide.
Commonly-employed ion-transmissive separator membranes or sheet materials, such as glass fiber mats, polyolefin membranes, and cast polymer and copolymer films, e.g., (poly)vinylidene fluoride hexafluoropropylene, serve equally well in the fabrication of rechargeable yttrium-ion battery cells of the present invention. Useful cell electrolytes likewise comprise compositions providing primary ion, i.e., yttrium, mobility. Such components are thus incorporated as 0.5 M to 2 M solutions of yttrium compound, such as yttrium trifluoromethane sulfonate, yttrium perchlorate, yttrium hexafluorophosphate, yttrium hexafluoropentanedionate, and yttrium hexafluoroborate, alone or in combination, dissolved in such proven stable electrolyte solvents as ethylene carbonate, dimethyl carbonate, vinylene carbonate, dimethyl ethoxide, tetrahydrofuran, propylene carbonate, and mixtures thereof.